Digital telecommunication systems typically require that the transmitter and receiver be synchronized precisely to the time and frequency alignment of the other system components. However, such synchronization may present difficulties for a system containing multiple transmitter and/or receiver components. For example, in a system where two digital mobile communications devices in motion are communicating with each other and a ground station through, for example, a satellite, if the ground station's zero time served as the absolute timing reference, the timing relationship between all components the system would now be known. However, the frequency relationship would be unknown due to the Doppler shift of the frequency caused by the motion between the two terminals. Thus, different frequency offsets would exist between the components of the system. These frequency offsets may present problems when switching communications between the different relative frequencies of the components as insufficient time may be available to compensate for the differing offsets utilizing conventional techniques.
Another example of a system which may have difficulties resulting from multiple frequency offsets is a mobile station which monitors a broadcast control channel from one station for control information, communicates with a second station handling voice/data traffic, and reverts back to monitoring the original broadcast control channel from the first station after a call. In such a system the switching between the control station and the voice/data station may result in information loss as a result of the time required for such a switch utilizing conventional techniques.
A further example of a system which may have difficulties associated with multiple frequency offsets is a satellite communication system. In satellite-cellular communications systems, such as the ACeS (Asian Cellular Satellite System), an additional frequency shift is applied to the system from the motion of a geosynchronous satellite in an inclined orbit. Ground stations in this type of system communicate with the mobile station (user terminal) via the satellite, which causes a different Doppler shift for each ground station due to the different geographical locations of the ground stations.
The user terminal in ACeS will, depending on its mode of operation, experience multiple Doppler frequency shifts. When in Idle mode, which is when the user terminal is monitoring the system for pages, the Network Control Center (NCC) is monitored through the satellite. When a call is initiated between a user terminal and the Public Satellite Telephone Network (PSTN), the user terminal synchronizes to a gateway (GW), which will handle the signaling and voice/data communication with the ground station associated with the PSTN. A user terminal to user terminal connection through the satellite between a first user terminal and a second user terminal requires yet another synchronization process. The call is established initially between each user terminal and the gateway. When the voice channel is allocated each user terminal synchronizes to the other.
The voice channel has two logical channels: S-SACCH (Satellite-Slow Associated Control Channel) and S-TCH (Satellite-Traffic Channel). The S-TCH carries voice through the satellite from user terminal to user terminal while the S-SACCH is connected to the gateway so the system can monitor the quality of the call. Thus, the user terminal, typically, must tune to one frequency for the S-TCH, characterized by the Doppler shift caused by the satellite moving relative to the other user terminal; and a second frequency for the S-SACCH characterized by the Doppler shift caused by the satellite moving in relation to the gateway. Also, the user terminal must be able to return to the Network Control Center frequency without delay when the call terminates.
Conventional methods for synchronization to a frequency typically involve a time and power consuming synchronization process involving multiple auto-correlations against a given bit-pattern which is performed on each frequency each time it is to be monitored. This synchronization process determines the frequency offset. Such a synchronization process is illustrated in FIG. 1. As seen in FIG. 1, the conventional synchronization process is a two-pass process with the second pass fine tuning the values from the first pass. The conventional synchronization process may begin by capturing 4 synchronization bursts sampled with a double wide window (i.e. a burst of B width is sampled in a 2B window). The synchronization bursts include known sequences from the network control center (block 100). An autocorrelation is then performed against the fixed pattern across 21 frequency bins with a window size of 30 bits (block 102). The timing and frequency are then adjusted (coarse synchronization) based upon the error resulting from the autocorrection determination (block 104). After this first pass, a second pass begins by capturing 4 more synchronization bursts utilizing a single width window (i.e. a window of B width) (block 106). The autocorrelation is then performed on these 4 synchronization bursts utilizing the fixed pattern across 21 frequency bins with a window size of 4 bits (block 108). The final timing and frequency adjustments (fine synchronization) are then made based upon the error resulting from the second autocorrelation process (block 110).
Typically, capturing four (4) synchronization bursts with the double wide window takes about 1.9 seconds, which is followed by another 120 milliseconds of Digital Signal Processor (DSP) processing. The second process using four (4) bursts with a single width window takes about the same amount of time. The complete synchronization process may take about 4 seconds for burst capture plus processing time. This process generally requires too much time to resynchronize when returning to the control channel from a traffic channel or to synchronize when switching between settings for a ground station versus a base station.
In present digital cellular systems, the user terminal tunes its oscillator to each new frequency after. completing each call. Thus, for a user terminal to base station call where the base station is a station other than the Network Control Center station, the user terminal would have to go through a synchronization process when returning to the Network Control Center station. Such a process may result in missed information during the lengthy synchronization process.
Furthermore, the synchronization process described above does not facilitate user terminal to user terminal communication because S-TCH and S-SACCH information may be spaced by as little as 900 microseconds while resynchronization between frequencies may require several milliseconds using conventional techniques. Thus, information may be lost during the resynchronization.
A further problem may arise as a result of Global System for Mobile Communications (GSM) and ACeS standards which both require that all timing be derived from a single oscillator. Due to frequency-time relationships, it may not possible to readjust the oscillator in a timely manner to account for frequency shifts when communicating with devices at two different locations having slightly different frequency shifts due to the Doppler effect or other frequency shifting mechanism.
In light of the above discussion, a need exists for improvements in the synchronization of multiple frequencies in a communication system.